//获取DIV
var nav = document.querySelectorAll(".nav > li");
var main = document.querySelectorAll(".main > div");
//for循环
for(var i = 0; i<nav.length; i++){
    nav[i].index = i;
    nav[i].addEventListener("mouseover",function(){
        for(var j = 0; j<nav.length; j++){
            nav[j].classList.remove("active");
            main[j].classList.remove("active");
        }
        console.log(this);
        this.classList.add("active");
        console.log(this.index);
        main[this.index].classList.add("active");
    })
}